1. Field of the Invention
The present invention relates to a method for continuous heat treatment of a liquid the flow of which varies, at which the liquid is first prewarmed by already heat treated liquid and thereafter heated to a predetermined temperature by a medium giving off heat in a second step.
2. Description of the Prior Art
When machine milking cows the flow of milk varies strongly. The variation is caused partly by the construction of the milking plant and partly by the physiological properties of the cows.
Milking plants work with vacuum and the milk is transported through a pipe line system which has a mainly equal underpressure to a container, a so called end unit.
From the end unit the milk is pumped under atmospheric pressure through a pipe line to a cooling tank or some other storage vessel.
Usually the pump of the end unit starts when the level in the end unit reaches an upper limit level and stops at a lower limit level. The pump of the end unit consequently transports the milk in batches. The amount may vary between 10 and 20 liter for each batch and the pump starts about once per minute.
The cows in large herds are usually milked in two groups of six to twelve cows in each group. The collected milk flow varies with the time since the cows give a larger amount of milk at the start of the milking than at the end of the milking.
The separate cow gives at each milking a total amount which is depending of how long time that has lapsed since the cow calved. In regions where the calving takes place during a certain part of the year one must count with large seasonal variations in the milk flow.
Even if the calving due to semine treatment is distributed more uniformly over the year the cows, which at the moment are high producing, are usually milked at first. The milk flow will then be considerably higher at the start of each milking than at the end when the cows which are low producing for the moment are milked. The time for milking a cow is not changed, however, with the amount of milk given from the cow.
When one wants to heat treat milk with an equipment which is directly connected to the milking equipment the heating equipment must therefore be constructed such that the treatment results will be equal even when the flow of product varies.
The variation in flow which depends on the face that the pump in the previously mentioned end unit works periodically may simply be removed by technical measures. The variation in flow which depends on the physiological properties of the cows must be accepted anyhow.
A simple solution is to collect all the milk in a storage tank. At a suitable time the heating equipment is started and the equipment then works at its own capacity, independently of the milking, in order to end the process soon after the end of the milking.
Such a way of carrying through the heat treatment is anyhow contrary to the general desire that the milk should not be stored uncooled during the two to three hours during which the milking takes place. The average holding time for the milk in the tank may be kept shorter, but there are still risks for an increase in microorganisms in the tank resulting in a decreased milk quality. Since the aim of the heat treatment is to increase the quality of the milk it should be very unfortunate, if the construction of the equipment should influence the milk negatively.
The heat treatment of for example milk at a varying flow of product gives rise to two separate problems.
The first problem is that the storing time of the product at the heat treatment temperature will be inversely proportional to the product flow when the heating equipment has a holding cell with a constant volume (length). Variation of the length of the holding cell during that treatment is found to be technically complicated. If a varying holding time is accepted, the heat treatment temperature may instead be adjusted to the holding time, i.e. to the product flow. A condition is anyhow that the change in the flow takes place slower than the change in temperature.
The system that controls the flow must in other words have a larger inertia than the temperature control. Of course it is possible to achieve such a control equipment using flow meters, a regulator and a control valve. It is found anyhow that the adjustment may easily be carried through by means of a pump with a variable speed control which consequently controls the milk flow. The pump treats the milk very gently and the rotation speed of the pump may be changed slowly in such a way that the relation between temperature and time during the heat treatment will be the desired one. The control signal to the pump equipment may also in an easy way be arranged to control the temperature.
The flow controlling pump may then be controlled continuously between zero and the maximal flow through the heating equipment or be made to work at a number of predetermined flow levels and corresponding temperature levels.
The other problem depends on the fact that the milk flow sometimes stops wholly due to the fact that a cow gives problem which results in a stop in the milking. During the stop the heating equipment works normally as long as there is milk available, but it then lowers the capacity and stops at the end. The milk which is then present in the holding cell will become colder in spite of the heat insulation around the cell. When the equipment is then started again colder milk than usual will pass the holding cell and will then in the regenerative section of the heat exchanger heat incoming milk. In order to achieve the right heat treatment temperature in spite of the low temperature an extra heat effect which is several times greater than during a usual operating step will be needed. The heat treatment is for control technical reasons usually electric and a large connection effect always gives troubles on a farm.
Another possibility is to give the end heater a large heat capacity in such a way that the instantaneous effect needed for restarting may be met with stored heat. Unfortunately this also results in a large inertia in the temperature control and makes the earlier mentioned adjustment of the temperature more difficult.